The manufacture of integrated circuits (ICs) requires many process steps which are executed with precision and accuracy. Precision is important because the ultimate dimensions of the components of integrated circuits are becoming smaller and smaller. Currently, some features have sizes on the order of less than one micron. Accuracy is important so that the related process steps are repeatable over time and produce results within a controlled range.
A significant part of many wafer manufacturing processes includes photolithography. Photolithography involves making an image of a part of the electronic circuit; rendering this part of the circuit onto a photographic plate, sometimes referred to as a photomask; and using the photomask and a light source to print that image onto a silicon wafer upon which a light-sensitive emulsion (e.g., a photoresist) has been applied. The exposed photoresist is developed to reveal the desired circuit elements. Other processes and treatments complete the structure of the layer.
An electronic circuit may have a number of photolithographic steps. The number of photolithographic steps often increases as circuits become more and more complex.
Two devices used for printing a mask pattern onto a silicon wafer are the projection aligner and the stepper. FIG. 1a shows one example of a projection aligner 100. One typical projection aligner 100, for example, includes two reflecting surfaces 106, 108. The light, typically supplied by a high energy source (not shown), such as a mercury lamp or a laser, passes through a point on a photomask 104 to a primary mirror 106. The light bounces off the primary mirror 106 onto a secondary mirror 108. From the secondary mirror 108, the light again bounces back to the primary mirror 106. The image on the photomask 104 is then projected onto a wafer 102. To assure accurate alignment, the aligner 100 relies upon receiving a reflected light signature of one or more alignment markers on the wafer 102 which can be aligned with corresponding markers on the photomask 104. In projection aligner printing, typically all of the product die on the wafer 102 are printed simultaneously. For example, if the wafer 102 has the capacity to hold 150 die, the mask has 150 images on it.
FIG. 1b shows one example of a stepper device. In using a stepper device, a pattern or mask 120 of a single layer of the semiconductor device 122 is placed on a reticle 124 and illuminated by a light source 126. A mirror 128 may be provided behind the light source 126 to reflect light back toward the wafer 122. The image on the reticle 124 may be two to five times (or more) larger than the final printed image on the wafer 122. The stepper optics 130 reduce the size of the reticle image to the final device size. The wafer 122 steps along and the aligner prints one die 132 at a time. Each die 132 may have an alignment marker to assure alignment throughout the building of the semiconductor device 122.
Each step builds an additional layer of the circuit upon the previously built ones. To assure that the layers line up with one another, the subsequent layers are printed relative to the first.
To properly pattern the photoresist material, it is often desirable that there be uniformity in the light which illuminates the photoresist material. Non-uniformities in the intensity of the illumination light may result in corresponding non-uniformities in the sizes and widths of device features. An exposure latitude is a measure of the amount of error allowed when forming the device features. This error arises from a number of sources other than the non-uniformity in the intensity of the illumination light including, for example, the reflection of light by the photoresist and layers on the wafer, as well as the amount of light absorbed by photoresist and the repeatability of energy for each exposure. Semiconductor devices formed according to 0.25 .mu.m design rules have exposure latitudes of 10 to 20 percent. However, for 0.18 .mu.m design rules, the exposure latitude may be 5 percent or less.
A typical lithography apparatus presently has an illumination uniformity specification of about 2 percent or less across the usable exposure field. This is a sizable portion of a 5% exposure latitude, given that there are other sources of error which are incorporated in the exposure latitude. Thus, by decreasing illumination non-uniformity, it may be possible to provide more accurate devices.
A large part of this non-uniformity arises from non-uniform or non-homogeneous characteristics of the lenses and other optical devices used in the photo lithography apparatus. These deviations result in an uncorrected error in printed feature size for any particular lithography tool. However, as feature sizes shrink, methods need to be found to reduce the errors caused by non-uniformity of the lenses and/or optical devices or those errors may contribute an increasingly significant portion of the allowed exposure latitude.